In a COT converter, a switch is turned on once a predetermined condition is detected. The on-state of the switch is maintained for a predetermined time period. At the end of the time period, the switch is turned off. Then the switch waits for a next cycle's turning-on when the predetermined condition is detected again. When the load of the converter increases, after turning off the switch, the output voltage decreases quicker and reaches the predetermined condition quicker, and accordingly, the switching frequency increases. A higher switching frequency leads to higher power loss. FIG. 1 shows a diagram illustrating the relationship between switching frequency and load under continuous conduction mode (CCM) in a COT converter. The switching frequency of the COT converter increases along with increasing load which is indicated by the output current.
Accordingly, an improved method is desired to at least overcome the above mentioned deficiency.